This invention relates to antennas. More particularly, this invention relates to a filtered antenna. Specifically, this invention relates to an antenna for marine band applications that filters out undesirable frequencies adjacent to the marine band.
It is well-known in boating that the marine VHF (very high frequency) band is centered around 156.8 MHz (megahertz)xe2x80x94VHF channel 16. Just above this band, at 157 MHz, are powerful land mobile paging signals. Andjust below channel 16, at 152 MHz are more powerful pager signals. With the increasing use of these pagers, reception of channel 16 by mariners is becoming increasingly difficult. These pager signals mix within the receiver section of most VHF radios and cause loud tones, whistles, and squawks.
For example, if there is a personal xe2x80x9cbeeper,xe2x80x9d a radio telephone paging transmitter, located in the area, transmitting at a frequency of 152.48 MHz, it will interfere with the safety calling information on channel 16 at 156.8 MHz. The telephone paging information (beeper) and the safety information are both received by the VHF antenna and passed along to the VHF transceiver. Depending upon the signal strength of the beeper signal, it can overpower the input receiving circuitry of the VHF-radio and cause interference in the form of shrill, squawks, squeaks, whistles, and tones that are at the least an annoyance and at the worst a safety hazard. This is especially problematic in big ship ports and in harbors located near cities.
One attempt to overcome this problem is to use externally mounted notch and band-pass filters. These filters simply plug in on the outside of the marine VHF and are pre-tuned to either pass the marine VHF band, or to notch the offending pager channel frequency. The band-pass filter is tuned specifically to the VHF marine band, and the notch filter to the local paging channel. These filter systems go in series between the marine VHF antenna and the VHF marine radio. These devices do not require any power and their passive design offers more than 40 db attenuation to annoying pager and land mobile interference. Unfortunately, these filters are quite expensive and, in some instances, cost more than the radio itself. Moreover, if the filters are incorrectly installed, they can actually decrease the sensitivity of the marine VHF communication system and reduce its performance.
Thus, the need exists for an economically manufactured antenna which significantly reduces interference from adjacent frequency bands and which provides superior performance at the preferred frequency band.
It is thus an object of the present invention to provide a marine antenna with an integral filter.
It is another object of the present invention to provide an antenna, as above, which is connectable to a receiver/transceiver for use with any desired frequency within the VHF band that covers the marine band from 156.050 MHz to 157.425 MHz including the distress, safety, and calling channel 16 (156.800 MHz).
It is a further object of the present invention to provide an antenna, as above, in which a filter is an integral part of the antenna for connection to the receiver/transceiver.
It is yet another object of the present invention to provide an antenna, as above, in which the filter includes a power matching feature that matches the impedance of the antenna with the impedance of the receiver/transceiver.
It is yet another object of the present invention to provide an antenna, as above, in which the filter provides a frequency suppression feature that minimizes reception of the undesirable frequency by the receiver/transceiver. It will be appreciated that the undesirable frequency is typically in a frequency band adjacent or very close to the frequency band that is desired to be received by the receiver/transceiver.
It is still another object of the present invention to provide an antenna, as above, wherein the filter provides a minimum of xe2x88x9215 db of isolation at the undesirable frequency; power matches the transceiver to the antenna with a maximum voltage standing wave ratio of 2:1 over the desired frequency band; and provides an insertion loss of xe2x88x920.5 db or less within the desired frequency band.
It is still a further object of the present invention to provide an antenna, as above, in which the filter includes a shunt circuit that includes a first variable capacitor serially connected to a first inductor, wherein the shunt circuit is connected across the first and second terminals of the antenna; and wherein the filter circuit further includes a power matching tee circuit that includes a second variable capacitor and a fixed capacitor serially connected to the first terminal, and wherein a second inductor is connected at one end between the second variable capacitor and the fixed capacitor and at opposite end is connected to the second terminal.
The foregoing and other obj ects of the present invention, which shall become apparent as the detailed description proceeds, are achieved by a filter for attenuating undesirable frequency signals between a transceiver and an antenna, includes a power matching circuit for matching the impedance of the antenna and the impedance of the transceiver, and a shunt circuit connected to the power matching circuit, the shunt circuit effectively forming a short circuit at the undesirable frequency to minimize reception of the undesirable frequency by the transceiver.
Other aspects of the present invention are attained by an antenna for minimizing reception of undesirable signals below a predetermined frequency band including a first and a second terminal connectable to at least a receiver, antenna components connected to the first and second terminals, and a filter integrally connected across the first and second terminals, the filter matching an impedance value of the antenna components with an impedance value of the receiver, and the filter effectively forming a short circuit at the undesirable frequency to minimize reception of undesirable signals below the predetermined frequency.
A preferred antenna incorporated in the concepts of the present invention is shown by way of example in the accompanying drawings without attempting to show all the various forms and modifications in which the invention might be embodied, the invention being measured by the appended claims and not by the details of the specification.